Phosphor particles are used in a variety of applications such as flat panel displays and decorations, cathode ray tubes, and fluorescent lighting fixtures. Luminescence or light emission by phosphor particles may be stimulated by the application of heat (thermoluminescence), light (photoluminescence), high energy radiation (e.g., x-rays or e-beams), or electric fields (electroluminescence).
Electroluminescent inorganic phosphors are of particular commercial importance. They are used in electroluminescent lamps, which in turn are used in, e.g., watches, clocks, and communication devices. The luminescent brightness and maintenance of the brightness of such inorganic phosphors are two important criteria for characterizing phosphor particles. Inorganic phosphors are subject to degradation which causes them to lose their brightness, and thus shortens their lifetime. Luminescent brightness is typically reported as a quantity of light emitted by the subject phosphor when excited. When reporting brightness (also referred to as luminosity), the conditions under which the phosphor is excited should also be reported. This is because the value depends on several factors including the voltage and frequency of the applied electric field and the temperature which the phosphor experiences. Maintenance refers to the rate at which inorganic phosphors lose brightness during operation. Water vapor is one of the most important adverse influences on maintenance. The effect of moisture or high humidity is referred to herein as humidity accelerated decay.
One way to protect inorganic phosphors and slow the rate of humidity accelerated decay is to encapsulate them with inorganic coatings, e.g., oxide coatings. Such coatings are generally transparent in order to prevent the loss of light emission by the inorganic phosphors and comprise oxides such as silica, titania, alumina, and mixtures of these. These coatings have been deposited by chemical vapor deposition onto phosphor particles in a fluidized bed. U.S. Pat. Nos. 5,156, 885, 5,418,062 and 5,439,705 (Budd) describe encapsulated electroluminescent inorganic phosphors which exhibit high initial luminescent brightness coupled with resistance to humidity-accelerated decay.